This invention relates to the field of printing inks, and in particular, to solvent-based inks for flexographic printing processes.
Printing inks such as flexographic inks are solvent-based inks applied by rubber rollers or pads to flexible sheets of plastic, foil, and paper. Many printing processes have been developed in the past and are available for printing on a wide number of surfaces. With the recent development of plastic films and the uses of such materials in packaging, flexographic inks have come into greater prominence because the equipment used for flexographic printing offers a useful and convenient method for printing on such plastic surfaces. Certain classes of plastic films are smooth, non-porous in the main, and very thin (of the order 50 to 76 micrometers thick). For these reasons, they do not have enough "give" for smooth printing from hard metal plates, and special flexographic printing from rubber plates is preferred. In addition, other classes of substrates, e.g. polymeric foams, are highly compressible, and printing from rubber plates by way of a flexographic printing process is preferred for this type of substrate.
The selection of resins or polymers as the binder for flexographic inks depends on the printing process, the solvent, and, most important, the substrate to be printed and the ultimate use of the printed matter.
Some of the desirable properties of a resin binder in flexographic printing inks are as follows:
(1) The resin should adhere well to plastic films to give high gloss, rapid dry, and a resistant attractive surface; PA0 (2) The resin should be stable on aging or chilling; PA0 (3) The resin should be compatible with the many modifying agents which may be used; PA0 (4) In inks which are applied with rubber rollers, such as flexographic inks, the resin should be soluble in solvents which do not attack rubber rollers. PA0 (a) a vehicle or binder, which comprises at least two resins which are compatible, one of said resins being a polyvinyl acetal or a dibasic acid-modified rosin ester, PA0 (b) a release agent, PA0 (c) a solvent for the vehicle and release agent, and PA0 (d) a colorant,
Polystyrene foam is frequently used as cap liners for containers, place mats, cartons for food, and the like. It is often desired by the manufacturer of the foregoing products to print trademarks, product information, advertising copy, or other information on these products. When ink is applied to the side of the foam which is to bear the information, transfer of ink to the reverse side of the foam can, and is likely to, occur under certain conditions of storage. This type of ink transfer is referred to as "set-off". Set-off can occur if the foam is stored in the form of a roll after printing. If the roll is tightly wound, the printed side of the foam contacts the reverse side of the foam under a high compressive force, resulting in transfer of a portion of the ink from the printed side to the reverse side. Set-off can also occur if printed foam sheets are stored in the form of a stack. Ink from the printed side of a given sheet can transfer to the unprinted side of the sheet immediately overlying it. Set-off may be aggravated when the printed side of the foam comes in contact with an adhesive-bearing surface. For example, polystyrene foam webs for container cap liners frequently bear an adhesive on the imprinted side. When stored in the roll, the adhesive has a tendency to pick off the ink from the printed side of the web. In some cases, the ink on the foam can be rubbed off under light or moderate rubbing friction. For example, the user of a foam place mat can easily rub off ink from the printed surface of the mat by brushing his hand over that surface.
A conventional inked surface may be overcoated with silicone release liner in order to prevent ink transfer. However, such a release liner is extremely expensive, e.g. the liner can double the cost of a printed article.